SWIFT (Sweep Imaging with Fourier Transformation) refers to a fast, 3D radial magnetic resonance imaging sequence including nearly simultaneous excitation and acquisition of spin data. In one example, SWIFT includes a gapped RF pulse. SWIFT (an example of which is represented in FIG. 1) includes a chain of gapped HSn low flip angle excitation pulses (including frequency sweep “f”) in the presence of a constant (per-view) readout gradient.
In one example of SWIFT, the HSN family of pulses are used for excitation. The RF-pulse can be defined using 32× oversampling. The gapping pattern is inserted into the pulse as blanking of the RF shape. The frequency response of the RF pulse depends on the gapping pattern for excitation. By way of examples, an RF duty cycle can be 12.5% (a baseband and a series of sidebands denoted as ±1, ±2, ±3, ±4, ±5, ±6, and ±7), 25% (a baseband and a series of sidebands denoted as ±1, ±2, and ±3), and 50% (a baseband and sidebands denoted as ±1).
An NMR signal is received in the gaps where the transmitter is gated off and the receiver gated on (doubly-gated). SWIFT has desirable features for a fast radial 3D sequence, including a high acquisition duty cycle, immunity to gradient integral errors, extremely short T2 sensitivity, and very smooth gradient updates (quiet, low gradient hardware demand).
In SWIFT, k-space is sampled along a radial projection from the center (k=0) and extending outward. The time allocated to obtain each k-space point is almost constant for the entire k-space. In one example, SWIFT entails parallel imaging for radial acquisitions and includes suppressing streaking artifacts in an equivalent way to interpolation between spokes.
As shown, for example in FIGS. 12-17, an image is generated using SWIFT by sampling k-space. The k-space sampling for the acquired FOV is smaller than the spectral width. FIGS. 12-17 illustrate acquired data points and the gaps represent missing data points.
A sideband signal arising from a magnetic resonance imaging sequence can lead to artifacts and produce a degraded image. For example, the sidebands can contribute spectra which can overlap, or fold into, a main lobe, and thus lead to undesirable artifacts in the resulting image.
Baker, E. B., et al, in Time-Sharing Modulation at 200 kc Applied to Broad and Narrow Line NMR for Base-Line Stability, The Review Of Scientific Instruments, Volume 36, Number 10, pages 1495-1498, October 1965, refers to a time-sharing method of alternately pulsing an NMR transmitter and receiver to reduce base-line instability due to leakage signal. This paper refers to methods for increasing the repetition rates to 200 kc and higher, which displace the sidebands well away from the carrier, giving undistorted broad line absorption signals, and high resolution spectra without side band overlap (e.g., 19F spectra). Some undesirable features associated with field and frequency modulation are thus avoided. The method entails using two pulsers, fast diode switches, and a broad band power amplifier.
Additional publications that may relate to the present subject matter include the following:
Idiyatullin, D.; Corum, C.; Park, J. Y. & Garwood, M., Fast and Quiet MRI Using a Swept Radiofrequency, J Magn Reson, 2006, 181, 342-349;
Idiyatullin, D.; Corum, C.; Moeller, S. & Garwood, M. Gapped Pulses for Frequency-Swept MRI, J Magn Reson, May 2008;
Andersen P M, Rosen M E, Erhard P, Adriany G, Strupp J P, Salmi R J, Ugurbil K Over-sampling digital IF receiver for MRI Proc of Int'l Soc of Magn Reson in Med, New York, N.Y. 1996
S. Moeller, C. Corum, D. Idiyatullin, R. Chamberlain, M. Garwood, ISMRM, Toronto 2008;
Curt Corum, Djaudat Idayatullin, Steen Moeller, and Mike Garwood. Doubly-Gated (Excitation and Acquisition) Effects in SWIFT, ISMRM Workshop on Data Sampling and Image Reconstruction, 2009;
Corum, C. A.; Idiyatullin, D.; Moeller, S. & Garwood, M. Signal Processing and Image Reconstruction for SWIFT, Proceedings 15th Scientific Meeting, International Society for Magnetic Resonance in Medicine, 2007, 1669;
PCT Pub. No. WO 2011/123657, entitled PULSE GAP CYCLING FOR IMPROVED SWIFT, International Application No. PCT/US2011/030744 Publication Date 06.10.2011, International Filing Date 31.03.2011, refers to a magnetic resonance image produced by shifting a gap during acquisition of spin data for a specimen. The spin data is generated by a gapped excitation sequence.